Fatigue failure of ball joint heads connected to the weight holder mechanism in railway applications

Abstract

Abstract The premature causes of failure of ball joint heads attached to the weight bearing bar used on high speed trains, which were continuously under traction, have been studied, with an average load of 47.5 kN plus a cyclical load of\xa0±\xa07.5 kN. Ball joint heads made of C45 (AISI 1045) carbon steel show a pearlite microstructure (80–90 %) with ferrite surrounding the prior-austenite grain boundaries. Two different external elliptical-shaped fatigue cracks of 14.5\xa0mm and 7.7\xa0mm nucleated in opposite positions on the cylindrical surface of the threaded shank prior to catastrophic cleavage failure. The Griffith equation applied in this study gave a value of 37\xa0MPa for the fracture stress (σf) produced by the largest crack, which multiplied by the section of the thread is equivalent to a load of 37 kN, similar to the applied service load. In addition, the critical stress intensity factor (KIc) calculation yielded a value of 8.8\xa0MPa\xa0m1/2, this lower KIc value was determined to be responsible for the large fatigue crack produced and the cleavage fracture causing the C45 carbon steel bolt joint failure. The pearlite/ferrite microstructure contributed to crack growth, since this type of microstructure in high carbon steels has lower strength and lower toughness than a tempered martensite microstructure. Using Paris equation, the cycles to failure were 1.5\xa0×\xa0105, which in service lifetime was an approximated 6\xa0×\xa0106 km for the piece to finally fail.